The present development of electronic circuits shows a growing tendency to effect digital functions and analog functions on the same circuit. Although bipolar technologies are proving more attractive for purely analog circuits, MOS technologies have the advantage when the digital part of the circuit is important. Among the latter, complementary MOS (or CMOS) technology offers, in addition to the advantage of a high integration density, the possibility of very low power consumption.
Most circuits comprising an analog part require the construction of a block delivering a reference voltage. Such blocks have already been proposed in CMOS technology and are most often derived from circuits known in bipolar technology by the name of band-gap voltage references. These circuits use a pair of transistors working at different current densities and which, while having a bipolar functioning characteristic, are compatible with CMOS technology. Such transistors, also known as substrate transistors, always have their collectors connected to the substrate, which limits their application especially when the user wishes to adapt circuits which have been developed in bipolar technology.
An example of a circuit using such transistors can be found in the article by R. Ye and Y. Tsividis entitled "Bandgap voltage reference sources in CMOS technology," which appears in Electronics Letters of 7th Jan. 1982, Vol. 18, No. 1. The reference voltage is obtained by effecting a linear combination of the base-emitter voltages of substrate transistors in such a way as to compensate the effects of temperature. This linear combination is achieved by means of an operational amplifier and resistors. When the operational amplifier is constructed with the help of MOS transistors it presents a large input offset voltage which, not being proportional to the absolute temperature, cannot be easily compensated. This offset voltage results in an imprecision of the reference voltage value of the order of 50 millivolts. The article by Bang-Sup Song and Paul R. Gray entitled "A precision curvature-compensated CMOS bandgap reference," which appears in the IEEE Journal of Solid-State Circuits, Vol. SC-18, No. 6, December 1983, shows how this offset voltage can be compensated by means of switched capacitor circuit techniques. However, the precision of the output reference voltage remains limited by the phenomena of charge injection produced by the transistors functioning as switches.
A new type of MOS transistor, which exhibits a bipolar functioning characteristic without having the limitations of substrate transistors, is described in European Patent Application No. 0093086, filed by the present applicant on 22nd Apr. 1983. This new type of transistor, which will subsequently be referred to as a compatible bipolar transistor, has already been used in creating a voltage reference source, as it would appear from FIG. 20 of the article by E. Vittoz which appears in the IEEE Journal of Solid-State Circuits, Vol. SC-18, June 1983 and which is entitled "MOS transistors operated in the lateral bipolar mode and their application in CMOS technology." The disadvantage of the circuit described in the aforesaid article resides in the fact that it does not take into account the finite current gain value of compatible bipolar transistors, nor its dependence on temperature. Another disadvantage of this circuit is the high output impedance value, which prevents a current from being drawn, especially to supply other circuits, without the reference voltage value being distorted.